Microwave switches are often used in satellite communication systems where performance, reliability and lifetime of system components are important. These parameters relate to the contact resistance of a microwave switch. In particular, microwave switches require low DC contact resistance, low insertion loss (i.e. the attenuation between the input and output ports of an activated path) and low impedance mismatch for good RF performance. Heat dissipation and insertion loss due to conductor and reflection losses increase for microwave switches with increased contact resistance. Furthermore, the life of a microwave switch is expressed as the number of actuation cycles for which the contact resistance does not deteriorate above a certain limit.
A microwave switch contact involves the physical engagement or contact of a first contact member by a second contact member. As it is known to those skilled in the art, the first contact member is a fixed contact also known as a probe and the second contact member is a moveable contact also known as a reed. The contact resistance and the life of the microwave switch are determined by the regions of the reed and probe that come into contact with each other (hereafter referred to as contact regions). The contact interface is herein defined as the surface of the contact members that are in physical contact with one another. To reduce ohmic losses, each contact member is typically plated with a conductive material having a high electrical conductivity like a metal such as gold.
Most prior art microwave switches have probes and reeds with contact regions that are flat surfaces. However, it is not preferable to use flat surfaces for both contact regions since there is a high degree of stress at the edges of flat contact regions. This stress may result in the excessive plastic deformation of at least one of the contact regions in which the yield strength of the material is exceeded. This in turn increases the contact resistance and decreases the lifetime of the switch contact members.
In order to address these issues, contact theory and the electrical junction between the probe and reed contact regions must be examined. The electrical junction comprises a plurality of spots, known in the industry as a-spots, that provide a multitude of parallel, microscopic electrical and mechanical connections between the probe and the reed contact region. The number and shape of the a-spots depend on the surface roughness of the contact regions and the contact pressure. The a-spots are located in clusters having a position and a diameter that is determined by the radii of the contact regions, the material properties (i.e. modulus of elasticity and Poisson ratio), large-scale waviness of the surface of each contact region and the contact pressure distribution. The contact pressure distribution is the distribution of the contact force on the contact interface. Although mechanical contact occurs at many a-spots, electrically conductive a-spots do not occur at surface insulating layers such as oxide films. Accordingly, the total contact resistance is a summation of bulk resistance, constriction resistance (i.e. the resistance of the a-spots) and film resistance (due to surface films and other non-conducting contaminants in the contact interface) (P. G. Slade (1999), Electrical Contacts: Principle and Applications, pp 4-15).
Research has shown that for contact surfaces having an anisotropic micro-topography, the a-spot distribution has an elliptical shape with a spreading resistance that is given by:                                           R            s                    ⁡                      (                          a              ,              b                        )                          =                              ρ                          4              ·                              a                c                                              ·                      f            ⁡                          (                                                a                  b                                            )                                                          (        1        )            where a and b represent the semi-axes of an elliptical a-spot, ac is the radius of a circular a-spot having an area identical to that of the elliptical a-spot and the function f is a form factor related to experimental data. The form factor decreases from one to zero as the aspect ratio (a/b) increases from one towards infinity (P. G. Slade, Electrical Contacts: Principles and Applications, New York, Marcel Dekker, Inc., pp. 4-15, @1999). The spreading resistance is half of the contact resistance in the absence of insulating films between the reed and probe contact regions.
Contact theory describes three different types of contact interaction from a mechanical point of view: incomplete mechanical contacts, complete mechanical contacts and receding mechanical contacts (K. L. Johnson, Contact Mechanics, Cambridge University Press, @ 1985). Incomplete mechanical contacts comprise non-conformal contact members (i.e. contact members which do not have identical contact regions). When the contact members are pressed together, the area of the contact interface increases in size as the applied contact force increases. The initial contact is made at a point or a line, which then increases into a curvilinear region as the applied contact force increases. The contact pressure approaches zero at the edges of the contact interface. Consequently, the a-spot clusters are located towards the center of the contact interface and the contact resistance is independent of the distribution of the a-spots (J. A. Greenwood, “Constriction Resistance and the Real Area of Contact”, Brit. J. Appl. Phys., 3,277,1970; M. Nakamura et al., “Computer Simulation for the conductance of a contact interface”, IEEE Trans. Comp. Hybrids Manuf. Technol., CHMT-9, p. 150, 1986; I. Minowa et al., “Conductance of Contact Interface depending on Location and Distribution of Conducting Spots”, Proc. Electrical Conference on Contacts, Electromechanical Components and Their Applications, p. 19, 1986). This is beneficial for having a consistent contact resistance that is fairly stable during a plurality of contact actuations. This is also beneficial for manufacturing batches of probes and reeds which all have a relatively similar contact resistance that is predictable. Furthermore, for contact members with non-conformal contact surfaces, Hertz theory predicts that the maximum of the contact stress occurs at a certain depth from the surface of the contact interface.
Complete mechanical contacts comprise contact members having conformal surface geometries (K. L. Johnson, Contact Mechanics, Cambridge University Press, @ 1985). Consequently, the contact pressure has a singularity (i.e. the stress magnitude is extremely high) at the edges of the contact interface. This may lead to excessive plastic deformation in the regions of the contact members situated in the vicinity of the edges, which reduces the lifetime of the switch. Furthermore, in a complete contact, the a-spots are distributed close to the periphery of the contact interface. Consequently, contact resistance is no longer independent of the distribution of the a-spots. Accordingly, the contact resistance may vary across consecutive contact actuations and is sensitive to manufacturing variability. This results in a degradation of the RF performance of the microwave switch.
Receding mechanical contacts comprise contact members having surface geometries that, when pressed together, result in a contact interface having an area that decreases when the applied contact force increases (Hill, Mechanics of Elastic Contacts, Butterworths-Heinemann Ltd., @ 1993). A receding contact is specific to thin membrane contacts having a low stiffness. Receding mechanical contacts are usually not applicable to microwave switches due to their low stiffness.
Prior art attempts to address the issue of contact resistance involve using probes and reeds that have conformal contact regions (such as two flat surfaces). Unfortunately, contact members with conformal contact regions behave as complete mechanical contacts. This is disadvantageous for the reasons specified above. Furthermore, this structure for the reed and probe contact regions does not allow for controlled wiping. Wiping involves cleaning the surface of the probe and reed contact regions from minor films and brushing aside particulate contamination. This is beneficial since minor films and non-conducting particles on the contact interface increase contact resistance. Accordingly, wiping will reduce contact resistance and improve contact performance (K. E. Pitney, NEY Contact Manual: Electrical Contacts for Low-Energy Uses, The J. M. NEY Company, @ 1973).
Another prior art method to improve contact resistance involves using texture features for the contact region. It is well known to those skilled in the art that a very low and consistent contact resistance may be obtained by imposing a surface texture having a roughness on the surface of the harder plated layer of the probe, for example. The roughness has a certain lay, which provides for elliptical a-spots when the contact regions of the probe and the reed are in contact with one another. In this case, there is a reduction in contact resistance because the a-spots have an elliptical shape with a high aspect ratio (i.e. the semi-axis length b is much larger than the semi-axis length and contact resistance decreases due to the effect in Equation 1). Furthermore, the contact interface area is larger since the softer plated layer on the reed (usually) contact region deforms around the asperities (i.e. microscopic surface peaks) of the harder plated layer on the probe contact region. In addition, an optimal surface texture may locate the a-spot clusters near the center of the apparent contact area. However, it is difficult to repeatably manufacture the surface texture on the probe since the surface texture and the lay direction are difficult to specify and measure. Accordingly, the contact resistance varies across different manufactured batches of switches. Furthermore, the contact regions of the probe and the reed form a complete mechanical contact, which results in a reduction in the life of the switch for the reasons specified above.